Vapor-liquid contacting devices, such as fractionation trays and packings, are employed to perform an almost endless variety of separations in the petroleum and petrochemical industries. For the purposes of this application, the terms “vapor” and “gas” are used interchangeably. Fractionation trays are used, for example, in the separation of many different hydrocarbons such as paraffins, aromatics and olefins. Trays are used to separate specific compounds such as different alcohols, ethers, alkylaromatics, monomers, solvents, inorganic compounds, atmospheric gases, etc. in the separation of broad boiling mixtures such as petroleum derived fractions including crude oil, naphtha, and LPG. Vapor-liquid contacting trays are also used to perform gas processing, purification, and absorption. A wide variety of trays and other contacting devices having differing advantages and disadvantages have been developed.
Fractionation trays and packings are the predominant form of conventional fractional distillation apparatus. They are widely used in the chemical, petrochemical and petroleum refining industries to promote vapor-liquid contacting performed in fractionation columns. The normal configuration of a fractionation column includes about 10 to 250 individual trays. Often the structure of each tray in the column is similar, but it is also known that the structures may alternate on vertically adjacent trays. Trays are mounted horizontally, typically at uniform vertical distances referred to as the tray spacing of the column. This distance may vary within different sections of the column. The trays are often supported by a ring welded to the inner surface of the column.
Fractional distillation has traditionally been conducted in cross flow or counter current contacting devices having an overall downward liquid flow and upward vapor flow. At some point in the apparatus the vapor and liquid phases are brought into contact to allow the vapor and liquid phases to exchange components and approach equilibrium with each other. The vapor and liquid are then separated, moved in the appropriate direction and contacted again with another quantity of the appropriate fluid. In many conventional vapor-liquid contacting devices, vapor and liquid are contacted in a cross flow arrangement at each stage. An alternative apparatus differs from traditional multi-stage contacting systems in that while the overall flow in the apparatus continues to be countercurrent, each stage of actual contacting between the liquid and vapor phases is performed in a co-current mass transfer zone.
During the fractional distillation process using conventional trays, vapor generated at the bottom of the column rises through a large number of small perforations spread over the decking area of the tray, which supports a quantity of liquid. The passage of the vapor through the liquid generates a layer of bubbles referred to as froth. The high surface area of the froth helps to quickly establish a compositional equilibrium between the vapor and liquid phases on the tray. The froth is then allowed to separate into vapor and liquid. During mass transfer, the vapor loses less volatile material to the liquid and thus becomes slightly more volatile as it passes upward through each tray. Simultaneously the concentration of less volatile compounds in the liquid increases as the liquid moves downward from tray to tray. The liquid separates from the froth and travels downward to the next lower tray. This continuous froth formation and vapor-liquid separation is performed on each tray. Vapor-liquid contactors therefore perform the two functions of contacting the rising vapor with liquid and then allowing the two phases to separate and flow in different directions. When the steps are performed a suitable number of times on different trays, the process leads to separation of chemical compounds based upon their relative volatility.
Many different types of vapor-liquid contacting devices including packing and trays have been developed as a result of the desire to improve equipment having this utility in the petroleum refining, chemical, and petrochemical industries. Different apparatus tend to have different advantages. For instance, multiple downcomer trays have high vapor and liquid capacities and the ability to function effectively over a significant range of operating rates. Structured packing tends to have a low pressure drop making it useful in low pressure or vacuum operations. Two very important characteristics of vapor-liquid contacting equipment in which improvement is always sought are capacity and efficiency. A co-current contacting device is believed to be one apparatus for achieving high capacity through using vapor-liquid separation devices such as demisters or centrifugal vanes for enhancing vapor-liquid separation at each stage. The co-current contacting device can also achieve high mass transfer efficiency through the co-current contacting of fine liquid droplets with vapor.
A co-current vapor-liquid contacting apparatus having a parallel arrangement is taught by U.S. Pat. No. 6,682,633 which discloses a modular apparatus for co-current contacting of vapor and liquid in a number of structural units which are placed in horizontal layers in a column or other enclosure. The structural units are horizontally spaced apart in each stage or layer to provide spaces for the downcomers from the modules of the next higher stage. The structural units of each stage are aligned parallel to the structural units in the superior and inferior stages. The downcomers deliver the liquid to contacting channels, with the contacting channels discharging the vapor and liquid into separation chambers at the top of a module. Vapor flows upward from the separation chambers to the contacting channel of the next higher module and liquid flows down through a single central downcomer to the next lower contacting channel.
U.S. Pat. No. 5,837,105 and related U.S. Pat. No. 6,059,934 disclose a fractionation tray having multiple co-current contacting sections spread across the tray. Liquid collected in a sump flows through a plurality of downcomers to the next lower tray where it is entrained in vapor rising through vapor openings of the tray and passed into one of two de-entraiment devices on the tray. The liquid from each de-entrainment device then flows into a sump. A number of arrangements are taught including parallel and non-parallel alignment of stages.
If maldistribution of liquid or vapor occurs in a vapor-liquid contacting apparatus having a parallel arrangement on adjacent stages, it is known that the fluid may not be readily redistributed along the length of the apparatus. Thus, maldistribution of liquid or vapor may propagate from one stage to the next, reducing the capacity and efficiency of the apparatus. Therefore, what is needed is a co-current vapor-liquid contacting device with an additional degree of freedom for fluid redistribution. In addition, the use of perforated decks in a relatively small area within the column may greatly increase pressure drop, even if the fractional open area is high. Therefore, what is needed is an improved co-current vapor-liquid contacting device with non-parallel stages and structures for transferring liquid from one stage to the next inferior stage without reducing liquid handling capability. Further, such a device with an optimum use of column space for fluid flow and contacting is needed for achieving high capacity, high efficiency and low pressure drop.